The present invention relates to a method and apparatus for encoding digital data for transmission or recording and also a method and apparatus for decoding the encoded signal produced by the encoder apparatus.
There are a number of methods and devices for encoding electrical signals for transmission over a long cable or for recording on a medium such as magnetic discs or tapes. Some of the more commonly known types of digital data encoding methods include the method of frequency-encoding digital signals such as described in U.S. Pat. No. 4,441,193. In these systems, a predetermined frequency signal represents the digital "1" and a different predetermined frequency signal represents digital "0". Through circuitry at the receiver, these various frequencies are separated from the transmitted signal and a digital signal is reconstructed. The receiver, however, must be synchronized with the transmitter so that the data information can be determined. One way of doing this is to provide a separate synchronizing clock signal in addition to the data signal being transmitted. The receiver recognizes this clock signal and clocks the receiving circuit to receive the incoming data.
Another method involves what is called phase encoding of data. This method of encoding is described, for example, in U.S. Pat. No. 4,532,559 to Long et al in which the transition from one level to another level, for example from a "0" to a "1", is used to represent data values rather than their absolute values. For example, an output signal transition in one direction from high voltage to low voltage is used to represent a binary "0", and a transition in the other direction, low voltage to high voltage, is used to represent a binary "1". An advantage of phase encoding digital type systems is that it is "self clocking". More specifically, it is not necessary for the transmitted signal to include a separate clock signal to synchronize the receiver with the transmitter. In a phase encoding method, the data transitions occur with sufficient regularity that a clock signal can be derived from the data being transmitted in a well known manner by using averaging methods.